1. Field of the Invention
The invention relates to an exhaust gas purification catalyst and more particularly to an exhaust gas purification catalyst in which a plurality of different catalyst layers are coated on a substrate.
2. Description of Related Art
Three-way Catalysts, which simultaneously carry out oxidation of the carbon monoxide (CO) and hydrocarbon (HC) and reduction of the nitrogen oxide (NOx) in the exhaust gas, are used as exhaust gas purification catalysts in automotive applications. In a widely available three-way catalyst, a catalyst layer made of a porous oxide, e.g., alumina (Al2O3), is coated on a cordierite honeycomb substrate and noble metal, e.g., platinum (Pt), rhodium (Rh), palladium (Pd), and so forth, is supported on this catalyst layer.
In addition, controlling the ratio between the air and fuel (air-fuel ratio A/F) supplied to the engine of an automobile to around the stoichiometric air-fuel ratio (stoichiometric) is critical for achieving the simultaneous and efficient purification of the three components CO, HC, and NOx by the action of the three-way catalyst. However, the actual air-fuel ratio varies to the rich side (fuel-rich atmosphere) or the lean side (fuel-lean atmosphere) around the stoichiometric due to, for example, the vehicle driving conditions, and as a consequence the exhaust gas atmosphere similarly also varies to the rich side or lean side. Accordingly, a high purification performance cannot always be maintained with just a three-way catalyst alone. Thus, in order to increase the exhaust gas purification capacity of a three-way catalyst by absorbing variations in the oxygen concentration in the exhaust gas, an oxygen storage material having a so-called oxygen storage capacity (OSC)—wherein oxygen is intaken when the oxygen concentration in the exhaust gas is high and oxygen is released when the oxygen concentration in the exhaust gas is low—is used in the exhaust gas purification catalyst; a ceria-zirconia (CeO2—ZrO2) composite oxide is an example of such an oxygen storage material,
However, exhaust gas regulations for, e.g., automobiles and so forth, have been becoming more and more rigorous throughout the world over the last few years, and responding to this has required additional improvements in the catalytic performance of exhaust gas purification catalysts. Thus, in order to bring about the effective manifestation of the functionalities of the aforementioned noble metals, oxygen storage material, and so forth, that are used in exhaust gas purification catalysts, exhaust gas purification catalysts have been proposed in which these materials are disposed on a substrate in a plurality of catalyst layers that are each different (refer, for example, to Japanese Patent Application Publication Nos. 2007-038072 (JP-A-2007-038072), 2006-326428 (JP-A-2006-326428), and 2010-005592 (JP-A-2010-005592)).
Various investigations have also been carded out into the material of the oxygen storage material itself. For example, because a CeO2—ZrO2 composite oxide having a pyrochlore phase has a high OSC, several proposals have also been made with regard to exhaust gas purification catalysts that use such a material (refer, for example, to Japanese Patent Application Publication Nos. 2003-246624 (JP-A-2003-246624) and 2005-170774 (JP-A-2005-170774) and WO 2008/093471).
It is very important for improving the exhaust gas purification performance of three-way catalysts that the catalyst metal that is the active metal species of the three-way catalyst, e.g., Pd, Pt, Rh, and so forth, be supported in a highly disperse state on the catalyst support. In the particular case of application as an automotive exhaust gas purification catalyst, the temperature to which the catalyst is exposed undergoes repeated variation between ambient temperature and approximately 1000° C., and the atmosphere to which the catalyst is exposed also repeatedly varies between a reducing atmosphere having a high concentration of HC and CO and a low O2 concentration and an oxidizing atmosphere having a low concentration of HC and CO and a high O2 concentration, Accordingly, it is also necessary to maintain the catalyst metal, e.g., Pd, Pt, Rh, and so forth, supported in a highly disperse state on the catalyst support under these conditions as well.
However, when these catalyst metals are subjected to long-term exposure to high temperatures, they migrate on the support and form coarse particles, that is, sintering occurs. A catalyst metal that has undergone particle growth due to sintering cannot maintain a high contact surface area with the exhaust gas, and the exhaust gas purification performance of the catalyst then ultimately declines with elapsed time. In addition to sintering-induced particle growth under high temperatures, the Pt and Rh are converted to the oxide and undergo particle growth in strongly oxidizing atmospheres, such as during fuel cut-off operation, while Pd also undergoes particle growth in a strongly reducing atmosphere, such as during acceleration, with the result that the exhaust gas purification performance of the catalyst ends up declining.
JP-A-2007-038072, JP-A-2006-326428, and JP-A-2010-005592 describe exhaust gas purification catalysts in which a plurality of different catalyst layers are coated on a substrate and also state that an excellent exhaust gas purification performance can be achieved with these exhaust gas purification catalysts even during, for example, cold starting and warm up. However, the inhibition of catalyst metal particle growth is not necessarily thoroughly examined in JP-A-2007-038072, JP-A-2006-326428, and JP-A-2010-005592, and there is thus still room to improve the exhaust gas purification catalysts described in this related art in terms of raising the exhaust gas purification performance.